Evaluation and assessment of the near field behind 3D printed-modulators for proton beam therapy

A 3D-printed modulator is an innovation developed for particle therapy delivery systems that enables a highly conformal and homogeneous dose distribution around the tumor within a very short irradiation time. In normal cases, the modulators are positioned far from the patient in order to avoid the field inhomogeneity resulting from the periodic structure of the modulators on patient’s skin. However, a smaller distance between the modulator and patient would provide a better dose conformation in the target volume. In this thesis, the FLUKA Monte Carlo simulation program was used to investigate the fluence distributions of protons penetrating through 3D-printed modulators and to determine the minimum distance at which the dose is homogeneous on patient’s skin. To implement the complex geometry of the 3Dprinted modulator in FLUKA, a dedicated FLUKA user routine with a shorter run time was developed. The sensitivity of the fluence ripple was also tested and found to be strongly dependent on the initial beam energy and the pin period of the modulator. The results of radiochromic film and dose measurements show a qualitatively good agreement with the FLUKA simulations. Furthermore, this thesis introduces the idea of the short distance of the modulator setup which could exploit strong dose inhomogeneities induced by the 3D-printed modulator for normal-tissue sparing. The minimum energy, which can utilize the advantages of the small distance setup without the interference of the dose inhomogeneity in the tumor, is 150 MeV for a tumor with a maximum width of 5 cm.